The osteoclast is the primary resorbing cell of the skeleton, but the mechanisms by which bone-seeking agents, such as parathyroid hormone (PTH), activate this cell are unknown. Such agents can theoretically stimulate resorption by 1) a direct interaction with osteoclasts, 2) enhancement of differentiation of osteoclast precursors, or 3) interaction with an intermediary (modulator) cell which can in turn stimulate osteoclast activity and/or promote osteoclast differentiation. Available evidence suggests that PTH does not directly interact with osteoclasts or their precursors and is therefore likely to enhance resorption via modulator cells. We propose that lymphocytes have the capacity to modulate PTH-induced bone resorption because 1) they produce an agent(s) (osteoclast activating factor) which stimulates osteoclasts, 2) lymphocytes bind PTH and are responsive to the hormone, and 3) bone resorption by isolated macrophages (surrogate osteoclasts) is not stimulated by PTH unless lymphocytes are also present. This proposal is directed towards examining the mechanisms by which PTH-stimulated lymphocytes enhance macrophage-mediated bone resorption. The initial studies will address the a) time course, b) dose dependency of the event, c) whether PTH induces blast transformation in lymphocytes, d) whether a short period of preincubation of lymphocytes with PTH induces these cells to stimulate macrophage-mediated resorption, and e) the interaction of other hormones with PTH in modulating MO-mediated resorption. We will next evaluate the alterations of macrophage physiology which occur when the cells are exposed to PTH-stimulated lymphocytes. These experiments will focus on properties of macrophages which are known to enhance their capacity to degrade bone matrix. These properties include a) enhanced differentiation of immature macrophages, b) enhancement of the capacity of resorptive macrophages to bind to bone, c) enhanced multinucleation of macrophages, and d) increased macrophage number. Finally, we propose to characterize the biochemical properties of the putative PTH-stimulated lymphocyte-produced factor(s) which promote bone resorption by macrophages.